Mont St Michel as a day trip?

This is doable as a day trip and there are companies offering tours; however, it will be about 12 hours with most of the time spent on a bus, then being dropped off on a very small island with hundreds, if not thousands, of other tourists. These tours are also quite expensive for what you get. If this sounds like something you'd like to do instead of enjoying "un cafe" at a Paris cafe or strolling along the Seine, go for it. Personally, I'm with your friend, spend the time in Paris, or do a day trip closer to Paris and accessible by public transportation, such as Chartres or Giverney.

It is "doable" as a day trip, but not recommended. it requires a train and a bus and takes nearly 4 hours each way:
Depart Paris Montparnasse 07:04, arrive Rennes 09:19 (TGV train)
Depart Rennes 09:40, arrive Mont St Michel 10:55 (local bus).
Just as long for the return journey. Last connection of the day: depart Mont St Michel 17:20, arrive Paris Montparnasse 21:11

That is why it is recommended as an overnight. There is lots to see in the area, it is good for car touring.
I agree with your friend. If you only have 1 week in Paris, stay there. Limit your day trips to nearer places, such as Versailles.

But, if you really want a day away from Paris, how about a day trip to London? 2 hrs. 15 mins city centre to city centre. Many companies offer this, with an orientation bus trip when you get there. Try googling "Day trip Paris London".

Doing Mont St Michel as a day trip from Paris ensures that you will arrive along with hundreds of other on bus tours or the public transportation options. It is a small island and the hordes detract substantially from the experience. I would stay in Paris instead.

Well, the Cluny Museum, Chateau Chantilly, Tour St. Jacques, Sainte Chapelle, Canal St. Martin, the Promenade Plantee, are all beautiful too. Have you seen them? There's more in town...

It is best to spend the night but will you be returning to this part of France again? Will you regret not going? You could rent a car at the end of the trip and drive and spend the night, then return car and fly home. It was a hassle for me to get there but well worth the effort....I enjoyed it far more than an extra day in Paris... I knew I might not have the opportunity again.

In October Mont St. Michel will not be as crowded as others make it sound.

Giverny may not be open in October.

If you know you will return to France, then skip it this time.

If you're not sure then do MSM as an overnight. It's a beautiful drive there and back and you'll be able to see MSM in the late afternoon as well as the morning.

As said before, MSM in late October will be pretty empty(empty compared to peak times). I did a trip in late October a few years ago and it was great. Honestly, I wouldn't miss a chance to see MSM, but I'm the type that doesn't mind packed schedules. You might enjoy getting out of the city for a day and just strolling around MSM.
BTW - if the tide is out enough their is a small chapel(St Aubert's) on the backside of the island(pretty cool). You can walk along the wall around to the chapel. Just watch out not to loss any shoes in the marsh(mud/marsh can suck a shoe away pretty fast).

Thanks everyone for your responses. An alternative daytrip as Chris F mentioned would be London but We just came back from London and am planning a longer stay hopefully next year.
I don't think I will be back to France for some time after this trip. I am now considering an overnight stay at MSM but am not sure how to plan the train and the bus transportation as well as the accommodation.. we will not be renting a car.

Sally,

I did an overnight trip from Paris to MSM with a friend this summer. I highly recommend doing it this way rather than as a day trip. We took a TGV train from Paris Montparnasse station to Rennes and then transferred to a bus for the 45 minute ride to MSM. The entire journey takes about 4 hours. It was easy and relaxing. The bus station is right outside the train station in Rennes. You purchase one ticket on RailEurope's website which includes the train and bus. We arrived at MSM mid-afternoon just when the hordes of day-trippers were leaving the island. We practically had the abbey to ourselves!! Our overnight stay was fantastic......the streets almost completely deserted and after dinner I walked back out on the causeway to see the magic of the island lit up at night. The sight was amazing! I was also able to experience the interesting sight of the islands many feral cats taking over the streets late at night, looking for food I would imagine. Cat fights ensued, which was interesting. We left the island the next day at about 11am....just when a new batch of day-trippers were descending upon the island. We stayed at the Auberge St. Pierre hotel which was very nice. Hotels on the island are not inexpensive but it is well worth the cost in my opinion. When I compared the cost of the train tickets, hotel and meals to the all day day tour from Paris there was not a huge difference in the cost and the overnight stay was so much more relaxing than a day trip would have been. Plus, on the day trip there is no way to avoid the crowds. I had always wanted to visit MSM and, to me, this was the only way to do it. You will not be disappointed if you stay overnight. Bon Voyage!